This invention relates to synthetic resin ophthalmic lenses, and more particularly to lenses having a high degree of surface hardness and photochromic properties wherein light absorbency increases as the ambient light is greater and decreases as the amount of ambient light decreases.
Synthetic resins have come into wide use as a safe material for spectacle lenses in place of inorganic glass since the adoption of standards by the United States Food and Drug Administration in 1972. Following that, the use of synthetic resins in spectacle lenses has also increased in Japan. The extent of use of the most popular synthetic resin for a lens, namely diethylene glycol bisallyl carbonate (hereinafter referred to as "CR-39") has not expanded to over 50% of the lens market in the United States. This is believed due to two disadvantages of CR-39 resin. First, the CR-39 lens will scratch or scar more easily than a lens formed of inorganic glass. Second, CR-39 lenses are not available with photochromic properties, except one model which changes color from a transparent to blue when exposed to light.
This presently available photochromic lens which turns to a blue color is not entirely satisfactory. This is because the absorbency, coloring speed and color fading speed are poor compared to photochromic inorganic glass lenses. The most popular photochromic lenses are those where the color changes from transparent to brown or from transparent to grey when the lens is exposed to light. Accordingly, it is fair to say that the available photochromic synthetic lens is not fully satisfactory.
There are many available techniques for providing photochromic ophthalmic lenses made of inorganic glass which have been commercialized. For example, U.S. Pat. No. 3,197,296 describes various photochromic ophthalmic glass lenses.
There have been several attempts to provide photochromic plastic lenses. For example, U.S. Pat. Nos. 4,049,846 and 4,110,244 describe absorbing silver halide into the swelled surface of a plastic lens. However, sufficient concentration of photochromic material cannot be absorbed and it is difficult to polymerize the surface completely. U.S. Pat. No. 4,170,567 describes utilizing various mercury containing photochromic materials with thermoplastic sheet materials. However, these teachings are not applicable to thermoset synthetic resins lenses, such as CR-39 and the like. This is due to the difficulty of dispersing the photochromic material to the casting monomers and oligomers. It is also not possible to produce multi-ply ophthalmic lenses practically in view of the high cost and techniques.
As noted above, it is difficult to provide synthetic resin lenses having satisfactory photochromic properties for the following reasons:
(1) The process of diffusing photochromic material into a base material and heat-treating cannot be carried out because of inferior heat resistance to synthetic resin lens compared to conventional inorganic glass lenses; and PA0 (2) Photochromic materials which can be diffused into synthetic resin base material utilized for ophthalmic lenses, such as CR-39 and the like are not available. This is due to the fact that photochromic materials are easily oxidized by the peroxide necessarily utilized as initiators in the casting process for forming the lenses.
Accordingly, it is desirable to provide a synthetic resin ophthalmic lens having improved photochromic properties. The lens must overcome the disadvantages of conventional plastic lenses, namely providing sufficient abrasion resistance, high solvent resistance, high impact resistance and have good tintability by a dispersed dye as well as providing the desired photochromic effects.